1. Field of the Invention
In steam turbines it is necessary to provide a sealed passage for steam between two separate casings or shells. Since the casings have different rates and magnitudes of thermal expansion, the seals must be capable of accepting vertical, axial, and transverse differential expansion of the shells, relative to the axis of the turbine shaft, without permitting significant leakage.
2. Description of Related Art
A common method of sealing employs a stack of piston rings that surround a pipe or snout that projects from one shell into a second shell. The stack of piston rings alternate in size, with somewhat smaller rings that fit tightly to the snout being held between larger piston rings that are held in the bore of the second shell such that the outer surface of the rings seal tightly to it. This method permits sliding motion of the pipe through the piston rings while maintaining a seal. It also permits transverse and axial motion of the pipe by sliding the small piston rings relative to the large piston rings, still maintaining a tight seal between the opposing large and small piston ring surfaces while the inner surface of the small piston rings and the outer surface of the large rings continue to seal tightly with the pipe and shell surfaces, respectively.
This method has been in use for a long period of time. It has several disadvantages:
The friction between rings is high and becomes higher as steam pressures are increased. This increases the stress in the rings when sliding motion is required and can cause permanent stretching of the rings that permits leakage.
The rings develop layers of oxide coatings in the hot stream environment that decreases the clearance between rings, eventually leading to a condition that locks the rings in a fixed position precluding motion without stretching the smaller rings, leading to leakage. The oxide layers can become torn and may be of varying thickness, also allowing leakage.
Oxide layers that form between the shell and the large ring outer surface lock these rings in position so that removal for cleaning is not practical without damaging the rings, thereby preventing further service.
Improved rings of U.S. Pat. No. 5,037,115 include a selected coefficient of expansion in combination with materials with low oxide forming ability. These rings have been effective but they normally require field machining of the rings as well as the snout and the bore of the shell.
The machining is expensive, time consuming and must be done with great precision.
In addition, the existing designs are handicapped by the shell surface that opposes the larger rings being a part of a thicker, more massive turbine shell structure, which tends during temperature changes to heat and cool less rapidly than the ring, thus causing either high stress on the ring or a gap between the shell and the ring. In addition, the shell most often has a lower steady state temperature than the ring which can also cause high stress on the large rings.
The invention eliminates previous problems by a combination of all or some of the following improvements.